AMPTE CCE Research Articles

Statistical analysis of compressional Pc3–4 pulsations observed by AMPTE CCE at L = 2–3 in the dayside magnetosphere

Few spacecraft observations of Pc3–4 (7–100 mHz) pulsations at L < 3 have been reported, although ground observations of the pulsations in the same L range are routine. In particular, there are no systematic spacecraft observations of compressional pulsations expected at the time of low‐latitude ground Pc3–4 pulsations that exhibit a constant frequency over a range of L. These pulsations have been attributed to fast mode waves trapped in the plasmasphere (referred to as plasmaspheric cavity mode waves) or to evanescent waves that exist inward of the turning point for fast mode waves propagating from the outer magnetosphere. We studied the properties of compressional Pc3–4 pulsations at L < 3 using magnetic field data simultaneously acquired by the Active Magnetospheric Particle Tracer Explorers Charge Composition Explorer (AMPTE CCE) and at the Kakioka ground station (L = 1.25) over a 2‐year period from 1985 to 1986. From time intervals when the satellite was on the dayside and between L = 2 and L = 3, we identified 63 cases of magnetospheric compressional (bz) Pc3–4 pulsations with or without an accompanying perturbation in the radial (bx) component, depending on the latitude of the satellite. The pulsations were observed primarily when AMPTE CCE was at prenoon hours (magnetic local time (MLT) = 0800–1200), and they were often accompanied by nearly identical magnetic pulsations at Kakioka. In an attempt to construct the spatial mode structure of the compressional pulsations, we have examined the relationship between the bx and bz components and also between the bz (or bx) component and the horizontal H (north–south) component at Kakioka. We find that the phase and amplitude relationship between bx and bz is consistent with waves standing in the meridian plane. The ground‐satellite cross phase is clustered near the values expected for radially standing or evanescent waves but also exhibits considerable deviations from the expected values. Although we have demonstrated that compressional Pc3–4 pulsations exist in the low‐L magnetosphere and that they give rise to pulsations on the ground, we conclude that further study is required to distinguish between the cavity and evanescent modes.

Ground‐satellite coherence analysis of Pc3 pulsations

Although Pc3 magnetic pulsations are commonly observed in the dayside magnetosphere and are believed to propagate into the inner magnetosphere and to the ground, how and where they establish themselves as a regular oscillation are not completely understood. In particular, it is not clear whether the cavity mode plays a significant role in determining the spectral properties of Pc3 pulsations. The mode, in principle, can be identified by multipoint observations of the spatial variation of the amplitude and phase of magnetic field perturbations, with at least one satellite providing evidence of compressional magnetic field oscillation in the magnetosphere. Motivated by these requirements for detection of the cavity mode, we surveyed combined magnetic field data from the Active Magnetospheric Particle Tracer Explorers Charge Composition Explorer (AMPTE CCE) satellite and the Kakioka ground station. We identified an interval on November 4–5, 1984, during which both the satellite (dipole L = 3–6) and ground station (L = 1.25) were on the morningside (near 8‐hour local time) with a small local time separation (within 0.9 hour) and observed Pc3 magnetic pulsations exhibiting a compressional component in space. Unlike previous studies, which used satellites located at L > 6, we observed high coherence in the Pc3 band between the compressional component (bz)in space and the horizontal component (H) on the ground. A cross‐phase analysis of the merged bz and H time series indicates that these components oscillated in antiphase over a range of satellite radial distance. We discuss whether the observations can be taken as evidence of a cavity mode oscillation in the magnetosphere.

Spacecraft transits across simulated field line resonance regions

The field line resonance (FLR) mechanism has become a cornerstone of magnetohydrodynamic wave theory in the magnetosphere and the solar atmosphere. Evidence for the mechanism comes mainly from auroral radar and geosynchronous spacecraft observations. Spacecraft in elliptical near‐equatorial orbits (particularly Active Magnetospheric Particle Tracer Explorers Charge Composition Explorer (AMPTE CCE)) have seen frequency‐varying oscillations of magnetic field lines but have not seen much evidence of the large‐amplitude, narrowband FLRs seen by auroral radars. We simulate the resonance region of a large event observed at GOES 7 and also of a smaller‐amplitude event we term a “typical Scandinavian Twin Auroral Radar Experiment (STARE) event” with reference to the STARE auroral radar results. One set of simulated narrowband FLRs is driven by a magnetopause Kelvin‐Helmholtz wave. We add a fundamental continuum oscillation and analyze the resulting time series using the AMPTE CCE time‐frequency analysis. We find that at the observed milliHertz frequencies the narrowband FLRs appear as large or small enhancements in the continuum of the azimuthal magnetic field but are not obviously “monochromatic.” For large events, enhancements in the spectrum of the compressional magnetic component may suggest a narrowband driver. We also analyze FLRs driven by waveguide modes and find no monochromatic signatures in the compressional magnetic field.

The spatial extent of radial magnetic pulsation events observed in the dayside near synchronous orbit

We have used simultaneous observations from the AMPTE CCE satellite, in an elliptical orbit with apogee at 8.8 RE, and GOES 5 and GOES 6, in a geostationary orbit at 6.6 RE, to investigate the radial and longitudinal extent of magnetic pulsation events with predominantly radial polarization. Twenty‐one events were selected by visual inspection of color‐coded Fourier spectrograms produced from data from all three satellites during a several month interval in fall 1984 when the apogee of AMPTE CCE was on the dayside; sixteen events were observed at all three satellites. Local time of the observed events ranged from 0900 to 1900 MLT, but the apparent longitudinal extent of the oscillation region varied considerably from event to event, ranging from the minimum resolution of 1.5 hours MLT (the local time separation of GOES 5 and GOES 6) to 8 hours MLT. Plasma wave data from AMPTE CCE indicated the waves occurred in regions of density characteristic of the outer plasmasphere (∼10 cm−3) and quite far outside the L shell region where densities reached 400 cm−3. These events occurred during magnetically quiet times usually after magnetic storms; interplanetary magnetic field data, when available, indicated an either roughly radial or northward orientation during the events. Wave onset often (but not always) occurred within one hour after sharp drops in the AE index to values of 100 or below. There was no apparent correlation of wave onset or amplitude with plasma beta, which ranged from 0.23 to 1.09 during the nine events presented here. Our frequent observation of the simultaneous onset of waves at different local times with considerably different frequencies reinforces the belief that the onset of these pulsations is determined by an instability that covers some longitudinal extent but that the frequencies are determined by local Alfven resonance conditions, not by the bandwidth of an external source. The data suggest that local plasma density increases associated with plasmaspheric refilling are the immediate cause of local instabilities leading to wave onset; the increase in density may alter the field line resonance conditions to allow the free energy of ∼ 100‐keV trapped ions to drive waves via the drift‐Alfven‐ballooning‐mirror mode instability.

Distribution of ULF energy (ƒ < 80 mHz) in the inner magnetosphere: A statistical analysis of AMPTE CCE magnetic field data

The distribution of ULF energy (frequency < 80 mHz) in the inner magnetosphere (L=2.5–6.5) has been studied using magnetic field measurements made with the Active Magnetospheric Particle Tracers Explorers Charge Composition Explorer (AMPTE CCE) spacecraft during its whole mission of over 4 years. The magnetic field time series was split into consecutive 10‐min segments and subjected to fast Fourier transform (FFT), and the spectral data were archived along with the Kp index and the satellite location. This data base was then used to examine the spatial distribution of ULF energy. Overall, the oscillation energy is localized to the dayside except for compressional oscillations associated with substorms. From the spatial distributions of wave power and spectral structures we identify several pulsation types, including multiharmonic toroidal oscillations; equatorial compressional Pc 3 oscillations; second harmonic poloidal oscillations; and nightside compressional oscillations. The toroidal oscillations are observable even in the plasmasphere, and using their frequencies we can determine the statistical radial profile of the plasma mass density and Alfvén velocity. We find a clear signature of the plasmapause in the profiles of these average parameters. Of particular interest among the other oscillation types are the equatorial compressional Pc 3 oscillations that have an average spectral peak around 30 mHz and are present in the prenoon sector and from L ∼ 3 to ∼ 5. The oscillations are similar to the compressional Pc 3 previously observed at and near geostationary orbit and might represent magnetohydrodynamic waves propagating from the parallel portion of the bow shock.

Pressure changes in the plasma sheet during substorm injections

Using data from the CHEM instrument on AMPTE CCE, data from the three‐dimensional plasma instrument and the SULEICA instrument on AMPTE IRM, and magnetometer data from both spacecraft, we have determined the particle pressure and total pressure as a function of radial distance in the plasma sheet for periods before and after the onset of substorm‐associated ion enhancements over the radial range 7–19 RE. We have chosen events occurring during times of increasing magnetospheric activity, as determined by an increasing AE index, in which a sudden increase, or “injection”, of energetic particle flux is observed. During these events the particle energy of maximum contribution to the pressure increases from about 12 to about 27 keV. In addition, the particle pressure increases, and the magnetic pressure decreases, with the total pressure only changing slightly. For radial distances of less than 10 RE the total pressure tends to increase with the injection, while outside 10 RE it tends to decrease or remain the same. Because the fraction of the pressure due to particles has increased and higher energies are contributing to the pressure, a radial gradient is evident in the postinjection, but not preinjection, flux measurements. These observations show that the simultaneous appearance of energetic particles and changes in the magnetic field results naturally from pressure balance and does not necessarily indicate that the local changing field is accelerating the particles. The changes in the total pressure outside 10 RE are consistent with previous measurements of pressure changes at substorm onset and can be understood in terms of the unloading of energy in the magnetotail and the resulting change in the magnetic field configuration. The increase in pressure at R ≤ 10 RE may also be due to the changing magnetic field configuration if in the region close to the Earth where the field becomes more dipolar, the angle at which the solar wind interacts with the magnetopause increases, while farther down the tail it decreases. Finally, the radial pressure gradient observed in the 7–12 RE region agrees very well with that necessary to maintain stress balance for an isotropic plasma.

A comparison of ULF fluctuations in the solar wind, magnetosheath, and dayside magnetosphere: 2. Field and plasma conditions in the magnetosheath

Data from three spacecraft (AMPTE IRM, AMPTE CCE, and ISEE 1 or 2) are used to study the correlation among the field and plasma conditions in the subsolar magnetosheath region, ULF wave activity in the magnetosphere, and the cone angle of the IMF. A disturbance parameter, R, which is the magnitude of the normalized resultant of unit vectors (calculated from measurements of the magnetic field or the plasma bulk velocity in a time interval), is used to describe the disturbance of the magnetosheath region. A “quiet” state has R values close to unity. We have studied five time intervals and found that when the R values of the magnetosheath magnetic field were below 0.8, indicative of a disturbed magnetosheath near local noon, transverse harmonic oscillations of magnetic field lines in the Pc 3, 4 range were observed in the magnetosphere and small cone angles were observed in the solar wind. We have also investigated the variation of other magnetosheath parameters (such as the magnetic pressure, the thermal pressure, the dynamic pressure, and the perturbation energy, etc.) under various magnetosheath conditions by comparing them with the disturbance parameter. It is found that the thermal beta (the ratio of the thermal pressure to the magnetic pressure) and the perturbation energy increase greatly as R decreases (i.e., as the magnetosheath region becomes more disturbed). The total pressure, which is the sum of the magnetic, thermal, and dynamic pressure of the subsolar magnetosheath region, decreases as the region becomes more disturbed. The dynamic pressure and the dynamic beta (the ratio of the dynamic pressure to the magnetic pressure) measured in the magnetosheath are poorly correlated with R, indicating that the changes in magnitude of the plasma bulk velocity in the subsolar magnetosheath have little effect on the occurrence of Pc 3‐4 waves in the outer magnetosphere. All magnetosheath parameters we examined became more variable during disturbed periods than during quiet ones. The implication of the above results to the transport of wave energy from the solar wind to the magnetosphere is discussed.

A comparison of ULF fluctuations in the solar wind, magnetosheath, and dayside magnetosphere: 1. Magnetosheath morphology

“Upstream waves,” generated in the solar wind upstream of a quasi‐parallel bow shook, are believed to be a major source of the Pc 3‐4 pulsation activity observed in the dayside magnetosphere. In an attempt to better understand the means by which “upstream wave” energy is transmitted from the solar wind into the magnetosphere, we compared simultaneous data from ISEE 1 and 2 in the upstream solar wind, AMPTE IRM in the subsolar magnetosheath, and AMPTE CCE in the dayside magnetosphere. Our observations indicate that dayside magnetospheric Pc 3‐4 pulsation activity and low IMF cone angles are associated with increased turbulence in the subsolar magnetosheath magnetic field (with large amplitude fluctuations both parallel and transverse to the average field direction), and with increased and highly variable levels of energetic magnetosheath particles. Fourier analysis of the magnetic field fluctuations shows broadband increases in wave power from 0.01 Hz to at least 0.5 Hz, but with peak power at Pc 3‐4 frequencies; there is no evidence in our data set of narrow‐band magnetic field variations in the magnetosheath at these times. Purely compressional waves, which are at times observed in the subsolar magnetosheath, have a somewhat narrower frequency distribution, but are associated with neither upstream wave activity nor magnetospheric pulsations.

Spatial variations in the suprathermal ion distributions during substorms in the plasma sheet

Using data from the Max‐Planck‐Institut für extraterrestrische Physik/University of Maryland Suprathermal Energetic Ion Charge Analyzer (SULEICA) on AMPTE IRM and the University of Maryland/Max‐Planck‐Institut für Aeronomie Charge Energy Mass Spectrometer (CHEM) on AMPTE CCE, we have determined the pre‐ and post‐injection Suprathermal energy spectra for the ion species H+, O+, He+, and He++ for six events in which substorm‐associated particle injections are observed in both the near‐Earth plasma sheet (7–9 RE) and farther down the tail (10–19 RE). We find similar spectral changes in both locations, with the spectra becoming harder with the injection. Post‐injection, the flux decreases exponentially with radial distance. Approximately the same gradient is observed in all species. In addition, we find that although the O+/H+ and the He++/H+ ratios increase with energy per charge, the ratios are approximately the same at the same energy per charge at the two spacecraft. The observations are difficult to explain either with a model in which the ions are accelerated at a neutral line and transported toward Earth or with a model in which the ions are accelerated in the near‐Earth region by current disruption/diversion and transported down the tail. In either case, the ions would have to be transported throughout the tail without much energization or deenergization in order to explain the energy per charge correlations. Further, earthward transport without energization would not lead to the observed radial gradient. A combination of these acceleration mechanisms, a disturbance that propagates throughout the plasma sheet, or a more global mechanism may explain the observations.

Multipoint observations of a small substorm

In this paper we present multipoint observations of a small substorm which occurred just after 0110 UT on April 25, 1985. The observations were made by spacecraft (AMPTE CCE, AMPTE IRM, DMSP F6, and DMSP F7), ground auroral stations (EISCAT magnetometer cross, Syowa, Narssarssuaq, Great Whale River, and Fort Churchill), and mid‐latitude stations (Furstenfeldbruck, Toledo, and Argentine Island). These data provide us with a broad range of observations, including the latitudinal extent of the polar cap, visual identification of substorm aurorae and the magnetic perturbations produced directly beneath them, in situ magnetic field and energetic particle observations of the disruption of the cross‐tail current sheet, and observations concerning the spatial expansion of the current disruption region from two radially aligned spacecraft. The DMSP data indicate that the event took place during a period when the polar cap was relatively contracted, yet the disruption of the current sheet was observed by CCE at 8.56 RE. We have been able to infer a considerable amount of detail concerning the structure and westward expansion of the auroral features associated with the event, and we show that those auroral surges were located more than 10° equatorward of the boundary between open and closed field lines. Moreover, we present evidence that the current sheet disruption observed by CCE in the neutral sheet was located on field lines which mapped to the westward traveling surge observed directly overhead of the ground station at Syowa. Furthermore, the observations strongly imply that disruption of the cross‐tail current began in the near‐Earth region and that it had a component of expansion which was radially antisunward.

A multisatellite case study of the expansion of a substorm current wedge in the near‐Earth magnetotail

This study presents observations made by four spacecraft (AMPTE CCE, AMPTE IRM, GOES 5, and GOES 6) and two ground stations (San Juan and Tucson) during a substorm that occurred at ∼0830 UT on April 19,1985. The spacecraft were arrayed in a configuration that allows for the examination of the spatial evolution of the substorm current wedge. CCE was located between the GOES spacecraft in longitude, but at a radial distance of 8.0 RE. IRM was located west of the other three spacecraft in the same sector as Tucson, but at a radial distance of 11.6 RE. The relative times at which the signature of the substorm current wedge was first observed at the GOES spacecraft and the ground stations are consistent with a simple longitudinally expanding current wedge. However, the times at which IRM and CCE observed the current wedge are not consistent with a current wedge that expanded only longitudinally. IRM first observed the signature of the current wedge at about the same time the signature was observed by GOES 6 and Tucson, and CCE observed the current wedge only after both GOES satellites and the ground stations had done so. Moreover, both GOES spacecraft observed signatures consistent with entry into the central plasma sheet before CCE and IRM did, even though we estimate that CCE was slightly closer to the neutral sheet than the geosynchronous spacecraft. The sequence of events suggests that during this substorm the disruption of the cross‐tail current sheet, the formation of the substorm current wedge, and the expansion of the plasma sheet began in the near‐Earth region, and subsequently spread tailward as well as longitudinally.

The energetic ion substorm injection boundary

The substorm injection boundary model has enjoyed considerable success in explaining plasma signatures in the near‐geosynchronous region. However, the injection boundary has remained primarily a phenomenological model. In this paper we examine 167 dispersionless energetic ion injections which were observed by AMPTE CCE. The radial and local time distribution of the events as a function of Kp is qualitatively similar to that envisioned in the injection boundary model of Mauk and McIlwain (1974). We will argue that particles observed during dispersionless injections are locally energized during the disruption of the cross‐tail current sheet. Therefore we identify the injection boundary, as derived from the spatial distribution of dispersionless injections, with the earthward edge of the region of the magnetotail which undergoes current sheet disruption during the substorm expansion phase. We will show that this qualitative model for the generation of the injection boundary can provide an explanation for the dispersionless nature, the double spiral shape, and the Kp dependence of the boundary.

On the relationship between the energetic particle flux morphology and the change in the magnetic field magnitude during substorms

In this paper we examine the relationship between the energetic particle flux morphology and the change in the magnetic field magnitude (Δ|B|) during substorms in the near‐Earth (<10 RE) magnetotail. We use Δ|B| as an indicator of position relative to the cross‐tail current sheet and analyze five dispersionless energetic ion injections which are representative of a set of 167 events. The five sample injections provide us with cases which occurred both above the plane of the current sheet and in the plane of the current sheet. It is found that the particle flux morphologies of the events are organized by the sign of Δ|B| into two generic categories. These categories are two ends of a spectrum of energetic particle behavior observed by AMPTE CCE. Events during which Δ|B| was negative exhibited large particle flux decreases and equatorward directed gradient anisotropies during the growth phase. These observations are consistent with a thinning of the near‐Earth plasma sheet. The injections were composed of an Earthward streaming leading edge, followed by a (relatively) isotropic hot plasma sheet. This is consistent with particle energization in the central plasma sheet during local substorm onset, followed by an expansion of the plasma sheet which engulfs the satellite. Events during which Δ|B| was positive exhibited a relatively small particle flux decrease during the growth phase, and the injections were short‐lived. The ions did not exhibit a strong earthward field‐aligned anisotropy at the leading edge of the injection, in contrast to the Δ|B|<0 events. During one of these events, which we believe was an in situ observation of the disruption of the cross‐tail current sheet, the largest energetic particle fluxes were associated with high levels of |dB/dt|. The observations made during this event are consistent with turbulent disruption of the cross‐tail current, and possibly a tailward motion of the region of particle acceleration. This event, and the Δ|B|<0 events, represent the extrema of energetic particle behavior. A second event during which the magnetic field increased, but by a small amount, exhibited energetic particle behavior which had characteristics of both categories. We believe that this case is an example of an event in the transitional range of the spectrum of energetic particle behavior during substorms in the near‐Earth magnetotail.

AMPTE/CCE observations of shell‐like He2+ and O6+ distributions in the magnetosheath

We present the first simultaneous observations of solar wind He2+ and O6+ shell‐like distributions in the magnetosheath. The He2+ and O6+ distributions are centered on the downstream H+ bulk velocity and have radii proportional to mass/charge. These observations are consistent with the electrostatic cross‐shock potential model discussed in the text.